Trump made the construction of the wall his signature issue in the presidential campaign

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U.S. Customs and Border Protection Acting Deputy Commissioner Ronald Vitiello shows a concept illustration of an enforcement zone of the U.S.-Mexico border during a news conference announcing the selected vendors for construction of the concrete wall prototypes for the border wall, Aug. 31, 2017 in Washington.

The Trump administration has taken another step toward building a wall on the U.S. border with Mexico, even as funding for the project remains in question.

U.S. Customs and Border Protection announced Thursday it will award contracts to four companies to build four prototypes for the wall.

CBP is providing few details of the concrete prototypes and says funding for four other prototypes for a see-through structure will be awarded next week. The prototypes will cost a total of $3.6 million.

Trump made the construction of the wall his signature issue in the presidential campaign. He promised that Mexico would pay for it, but Mexico has refused.

Trump is now demanding that Congress fund the wall and has threatened to shut the government if it doesn't do so.